Drill bits are a very common tool used in the construction and maintenance industries. There are a wide variety of drill bits that have been developed. Some drill bits are specific purpose drill bits while others are multipurpose drill bits.
The construction and maintenance industries involve the installation and maintenance of services such as wiring, plumbing and pipe fitting. Often these services are routed through structural members of a building. For example an electrician is commonly required to route wiring through wall studs, roof joists, floor joists and other structural elements. This installation process commonly requires the boring of holes through structural members.
A commonly used building material is wood. Wood is a nonuniform material and this non-uniformity can cause problems in the boring process. For example wood may have non-uniform density, knots, wood rot, and/or pitch pockets. In addition there may be nails or other fasteners that may interfere with the boring process.
Many different bit types may be used to bore a hole, each type uses a different mechanism and these mechanisms vary greatly. Typical boring bits that are generally available include a spade type bit, a hole saw type bit, a Forstner bit and an auger type bit. Problems that may arise when using these various types of bits include inadequate waste removal, limited self feeding action, binding of the bit in the bore, dulling of the cutting edges through wear and hitting nails, difficulty re-sharpening cutting edges, incomplete cutting or loss of operator control during some phases of the boring process.
The auger type drill has a number of advantages over the other drill bits. For example, the auger type drill bit typically has a self-feeding lead screw for urging the bit into the workpiece, a spur for severing the wood fibres to form the radius of the bore, a chisel cutting edge to lift the severed wood fibres, a helix shaped flute that proceeds up the body of the bit to provide a mechanism for waste removal.
However, there are a number of shortcomings of the existing wood auger type drill bits. These shortcomings include: the spur becomes dull and/or damaged or broken off; the chisel cutting edge becomes dull and/or damaged; the helix shaped flute is often too narrow and too shallow to sufficiently remove waste; and the cutting edges of the bit are difficult and tedious to resharpen accurately. Once cutting edges become dull, the bit fails to cut and causes excessive heat build up. Further, once cutting edges become dull or missing, performance drops and binding can occur.
Another major shortcoming of the wood auger bit is the incomplete cutting action that occurs as the bit emerges from the opposite (back) side of a workpiece. In use, as the self-feeding lead screw of the wood auger bit enters the workpiece a self-feeding action commences thus drawing the bit into the workpiece to such an extent that the spur scribes a circle in the workpiece around the point of entry and severs the fibres around this circle. The continuing rotation of the bit and subsequent advancement of the bit causes the chisel cutting edge to uplift the fibres previously severed, forming chips. Friction between the chips and the cylindrical surface of the created bore and the helical inclination of flute cause the chips to advance away from the chisel cutting edge and ultimately out of the bore. This process continues until the self-feeding lead screw emerges from the opposite (back) side of the workpiece. As the lead screw breaks through the opposite (back) side of the workpiece the self-feeding function of the lead screw is progressively lost and the bit is then forcibly advanced by the operator. Due to the loss of the self-feeding function, there is a substantial decrease in the cutting action and a subsequent increase in drill rpm. Depending on the resiliency and specific nature of the wood composition (knots or nails for example) at the exit location of the workpiece, a "D" shaped hole may result, with the self-feeding lead screw, the spur, the chisel cutting edge and a portion of the body of the bit extending out of the workpiece. Where a "D" shaped hole is formed, the higher rpm of the drill bit and the force of the operator may cause the leading edge of the flute to "ride" or travel briskly through the workpiece. This situation is most annoying and quite dangerous as the operator is not prepared nor able to resist this tendency.
Given a typical slope of a wood auger bit, an axial distance of approximately one inch is travelled per revolution of the bit, this coupled with an estimated 400 rpm of the bit will cause the bit, drill and operator to potentially be pulled toward the work at approximately 7 inches per second. Note this 7 inch travel is greater than the length of a typical auger bit itself. A drill operator is commonly unable to resist this pull, particularly in awkward drilling positions or where the axis of the bit does not extend through the centre of gravity of the operator as is often experienced in the field. At cutting speeds greater than 400 rpm these dangerous effects are even greater.
In order for the operator to finish boring this incomplete or "D" shaped hole, the operator must either reverse the wood auger bit manually or reverse the electric drill, if equipped with such a feature, and then attempt to redrill the incomplete hole in hopes of removing the remaining "D" shaped portion of the workpiece. This procedure is an unconventional use of the auger bit and may cause unpredictable actions as the bit may abruptly engage the remaining "D" shaped portion of the unfinished hole.
Therefore it would be beneficial to provide a bore forming tool that is capable of forming a generally circular opening upon exiting the workpiece and reducing the dangerous "pulling" tendency that exists with current auger bits. In addition it would be beneficial to provide a bore forming tool wherein the cutting edges can be easily replaced.